单选题 Among the most complexcrystals are that of silicon dioxide, which has seven different structures at various temperatures and pressures, the most common being quartz.
【正确答案】 B
【答案解析】答案：B 分析：改为those，指代crystals.后面的which指代silicon dioxide,所以用单数has 翻译：在最复杂晶体中有二氧化硅的结晶体，二氧化硅在不同温度和压强下有7种不通结构,最常见的是石英。